Influence of a Ni Metal Phase and Geometrical Structures of YSZ in Ni-YSZ SOFC Anode to Its Mechanical Properties

Abstract

Conventional SOFCs, which adopt an anode supported type cell design, use the porous cermet of Ni and YSZ as the anode. On the viewpoint of mechanical design, the anode is an important component next to the electrolyte because it prevents the electrolyte from destruction caused by thermal stress, chemical stress, and outer force. However, the anode also show uncertainly in the response to outer force, which is most fundamental property for mechanical design. This uncertainly is due to the existence of Ni metal phase and complex geometrical structure in the anode (Ni-YSZ). To maintain electrical conductivity, gas permeability, and catalytic activity, the anode consist of Ni, YSZ and porosity, which volumatic ratio are almost same, and each phase are continuously connected in the anode, in other words, three networks co-exist in the anode. At high temperature close to ½ of melting temperature of Nickel(Tm=1455oC), Nickel is easily deformed plastically. By contrast, YSZ(Tm=2700oC) is still brittle. In addition, each network are non-uninform. So, stress concentration and bending occur by outer force applying. These things make behavior more complex. However, the response of outer force and its relationship between the response to microstructure have not been well studied. Yield strength of nickel (ca.150MPa) at room temperature is close to fracture strength of YSZ(ca. 200MPa). Therefore, at room temperature, the anode cermet show brittle behavior because YSZ network is fractured at the same time of nickel network yields. However, at SOFC operating temperature, nickel yield strength becomes very low (ca.20MPa@800oC) . In this situation, nickel network yields much smaller stress than fracture of YSZ network. Geometrical structure in cermet is categorized to two types. First type is often used for high speed tools, and, guest material shows isolated distribution in host material. Second type is used for the anode, and host and guest materials make continuous networks in cermet, respectively. Response to outer force in first type are dominated by host materials. However, that in second type (SOFC Ni-YSZ anode) are not studied well.Therefore, we studied creep curves (response to small stress), stress-strain curves (response to large stress) in Ni-YSZ. Furthermore, we evaluated the influence of volume ratio between Ni and YSZ to fracture toughness. Based on the results, the influence of a Ni metal phase and geometric structures of YSZ to mechanical properties were discussed.Three chemical compositions (Ni vol% 30, 44, 58 : Ni30, Ni44, Ni58) of Ni-YSZ were tested. Both creep curves and stress-strain curves showed different behavior to outer forces from porous Ni metals or porous YSZ. In the case of creep test, activation of creep rate varied from that of YSZ to that of Ni metal, and this variation was suggested to be related to accumulated strain in Ni-YSZ specimens. Ni-YSZ specimens showed different fracture modes to outer forces from different directions. Under uniaxial compressive force, Ni44, Ni58 specimens was squished at the strain range over 1%, and Ni44 specimens were torn under bending force. These results suggested that mechanical properties were strongly influenced by the rigidity of YSZ network in Ni-YSZ. In addition, we discuss about the rigidity of YSZ networks at the presentation.